^Bruce L. Derwing, Royal Skousen, Productivity and the English Past Tense, in The Reality of Linguistic Rules, page 202

^Survival of the Strongest, in Studies in the History of the English Language V (2010, ISBN3110220326, page 101: What may come as a surprise, depending on the framework in which one operates, is that sit must have been largely responsible for the preterite shat of shit and probably the preterite spat of spit. Shit should conjugate shite~shote, and spit was originally weak (OED).